A light emitting diode (LED) is a semiconductor optical device capable of producing light in the infrared, visible or ultraviolet (UV) region. LEDs emitting in the visible and ultraviolet are made using gallium nitride (GaN) and its alloys with indium nitride (InN) and aluminum nitride (AlN). These devices generally consist of p and n-type semiconductor layers arranged into a p-n junction. In a standard LED device, semiconductor layers are evenly grown onto a polished substrate such as GaAs or sapphire. A typical semiconductor layer is composed of gallium nitride (GaN) that has been doped to be a p or n-type layer.
Important figures of merit for an LED are its internal quantum efficiency (IQE) and light extraction efficiency. For a typical LED the IQE depends on many factors, such as the concentration of point defect, Auger processes and device design. In the case of nitride LEDs grown along polar (0001) and (000-1) directions the internal efficiency is also reduced due to the distortion of the quantum wells between the n- and p-doped layers caused by the internal electric fields. The light extraction efficiency of standard LEDs based on GaN is determined from Snell's law to be 4% per surface. An LED commonly includes several quantum wells made of a small energy gap semiconductor (well) and a wider bandgap semiconductor (barrier). Visible LEDs employ indium gallium nitride (InGaN) as the well and GaN as the barrier. Ultraviolet LEDs employ AlGaN of different compositions as both wells and barriers. The IQE of an LED device based on nitride semiconductors grown along polar direction is reduced by electric fields across its quantum wells. This phenomenon is referred to as the quantum confined Stark effect (QCSE). The QCSE affects LED light emission by red shifting the emission wavelength and reducing photoluminescence intensity. The rather small value of light extraction efficiency in the standard LED is the result of the high refraction index of the semiconductor layer at the exit interface.
A number of approaches have been proposed to enhance the extraction of light from LEDs. For example, in GaAs LEDs, the extraction of light is affected by the absorption of the emitted light in the GaAs substrate. To mitigate this problem, one can use epitaxial lift-off and wafer bonding methods to transfer the GaAs LED structure to transparent substrates. Another approach involving the optimization of LED surface geometry (such as the truncated inverted pyramid), combined with the use of substrate mirrors, has pushed the extraction limit to 30%. Other approaches involve the use of a continuously variable refraction index transparent material to reduce the back-reflection at the interface. Some of these approaches have some manufacturing limitations and the last one suffers from fast index-material degradation with time.
An approach that is recently becoming increasingly attractive is photon extraction from randomly micro-textured thin film surfaces. It has significantly improved extraction efficiency, with record external quantum efficiencies of 44% demonstrated at room temperature for GaAs based LEDs (Windish et al., 2000). In this reference, the textured surface was formed after the growth of the LED using lithographic methods. It turns out that, even in that case, most of the photons are still extracted from within the emission cone inside the critical angle corresponding to a flat surface. Consequently there is still a wide room for improving extraction well beyond the present values.
Visible and UV LEDs based on GaN and other III-nitride materials are used widely for full color displays, automotive lighting, consumer electronics backlighting, traffic lights, and white LEDs for solid state lighting. A variety of approaches are used towards formation of white LEDs. One approach is the utilization of three-color LEDs (RGB) and an alternative approach using hybrid methods such as UV LEDs in combination with a tri-color phosphor or blue and blue/red LEDs with two or one color phosphor. Current white LED performance has reached 30 lm/W, while efficiency more than 200 lm/W is required for commercially attractive semiconductor lighting.
The current IQE for electron-hole pair conversion to photons of nitride LEDs is ˜21% (Tsao, 2002). Thus the IQE needs to be increased to 60%-70% for applications related to solid-state lighting. To accomplish this, a number of improvements in the current state of the art are required. For example, band-gap engineering (quantum wells, quantum dots) must be involved to optimize carrier-to-photon conversion. Also, improvements in the various layers of an LED structure are required to reduce the defect density and thus improve carrier transport to the active region. Such improvements reduce parasitic heating and lead to device longevity, enhanced color stability, and reduced consumer cost over lifetime.